The erosion or collapse of mountains and streams is caused by the downward travel of earth and gravel produced upon the collapse of mountains by mainly rainfall, snowfall, earthquakes, or the like, whereas erosion or collapse of ordinary rivers is caused mainly by floods. However, because there are many techniques common to the prevention of these types erosions or collapses, mountain conservancy work techniques for preventing land erosion and improving streams and for revetments conventionally applied to mountains or streams will be described below.
Obviously, the best practice for preventing such erosion and collapse and for restoration from such erosion and collapse is to make full use of the water retention and mountainside-stabilizing ability of forests by covering a mountain with a forest, thereby perfecting a mountainous and forested environment so as to permit prevention or alleviation of erosion and collapse of mountainsides and streams even when a heavy rainfall or snowfall occurs.
However since very great costs and a long period of time are required to condition and maintain such a mountainous and forested environment, many artificial works have been constructed at the same time that efforts have been made to perfect the mountainous and forested environments. These works are characterized in that an independent structure is installed at a prescribed location in a mountainous district, thereby it being attempted to reduce the velocity and quantity of flowing water, earth, and gravel by the effects of the weight of the structure, or the bearing capacity of the bedrock and the like supporting the structure. Installing a structure having these functions requires much material, labor, and costs. Their outlines are as follows (see Sadao Hagiwara, "Forest Water Utilization and Land-erosion Prevention". (Dendrology, Vol. 9), Asakura Publishing Co., 1953; and Aritsune Takei, Ed., Land-erosion Preventive Engineering, Eibundo Publishing Co., 1993).